Pistons for use in high performance internal combustion engines must be light in weight, resistant to high temperatures and strong. Carbon-carbon composite materials are tough, light, resistant to heat and have low coefficients of friction and thermal expansion. The demands of piston design and the qualities of carbon-carbon composites are therefore compatible as evidenced by structures such as those described in U.S. Pat. No. 4,683,809.
One parameter affecting several of the structural qualities of carbon-carbon composite is fiber orientation. When the composite structure is composed of randomly directed fibers (mat or N-D structure) the qualities thereof vary as fiber length and homogeneity vary. Failure to adequately control these additional parameters can induce errors which will negatively influence structural strength and consistency of manufacturing results.
When a composite structure is composed of randomly directed fibers, there is no way to increase strength in a specific area to accomodate localized stresses except to thicken the structure. This exacts a serious penalty in increased weight. Alternatively, the selective orientation of fibers prior to their embedding in the matrix serves to reinforce orientation of fibers prior to their embedding in the matrix serves to reinforce high-stress areas without greatly increasing overall weight.
Carbon-carbon composite structures are relatively expensive to form because of the high raw material costs of certain components such as polyacrilonitrile (PAN), plus the lengthy and energy intensive re-impregnation and re-pyrolization steps required to obtain a structure of sufficient strength and density to withstand the operating environment inside an internal combustion engine. The time required to complete this densification process increases exponentially as the sectional thickness of the composite structure increases.
Using a knitted fiber architecture, which enables the fiber directions to be controllable, results in increased and better controlled interlaminar strength properties than cloth or mat layups. Consequently, the increased strength realizes significant savings from more economical use of materials, decreased difficulty of machining, shorter fabrication times, and reduced energy consumption.
Therefore, an object of the present invention is to increase the strength per unit weight of a carbon-carbon composite piston by using knitted or warp-interlock pre-forms of structural shapes to control internal fiber orientation.
An additional object is to reduce manufacturing costs for a carbon-carbon composite piston by the use of near net shape knitted or warp-interlock pre-forms so as to reduce lay-up, material and molding expenses.
An additional object is to reduce manufacturing time and expense for a carbon-carbon composite piston by decreasing component thickness and thereby shortening the densification processes.